In a conventional engine, the accessory box containing fuel pumps, bearing lubrication pumps, hydraulic pumps for controlling various members, electricity generators, and the starter, is placed outside the engine and receives power taken from the engine by means of a vertical shaft and angle takeoff, in particular connected to the rotor and in particular the low-pressure rotor.
Over the years, the increase in the compression ratios and the temperatures at the inlet to the turbine, and the improvements in materials and efficiency have led to a constant reduction in the size of engines so as to obtain a thrust/weight ratio that is ever greater, whether for civilian applications and for military applications.
The system for taking off power and the accessory box have had difficulty in following this progress correspondingly, and they thus represent a large fraction of the volume and the weight of an engine, particularly of a low thrust engine, i.e. an engine of small size, and particularly when the accessory box, which is generally placed under or on the engine, and sometimes to one side of it, carries an air starter and an electricity generator that are separate.
The use of small engines that are ever simpler and less expensive for the purpose of propelling training airplanes, observation or attack drones, and cruise missiles, is requiring engine manufacturers to make such engines more furtive. This can be attempted by greatly reducing their frontal surface area, thus also achieving a significant reduction in drag, thereby very significantly increasing the flying time or the range of aircraft or remote-controlled vehicles fitted with such engines. In order to reduce the weight and the frontal surface area of engines, it therefore appears desirable to envisage integrating an electric generator-starter in the engine and to eliminate the use of mechanical connections, so that the interface between the engine and the accessories then relies on electrical transmission.
In wide-bodied aircraft, having ever more numerous electrical or electrohydraulic flight controls, and also in radar, advanced warning, and electronic surveillance airplanes, electrical power requirements are large. The engines of such aircraft are fitted with booster or auxiliary generators, thereby increasing the size of the accessory boxes and also their weight for supporting them. In an engine having a large bypass ratio, it is therefore advantageous to integrate an auxiliary generator, and more generally a generator-starter in the engine, in order to reduce the size and the weight of angle takeoffs, or indeed to eliminate them, and to obtain a thinner cowl, with some of the electrically-driven accessories being housed in the pylon.